A number of lifts have been developed in order to aid handicapped persons, in particular, wheelchair passengers, in boarding and exiting vehicles such as buses, trains, vans, and the like. One type of lift, commonly known as a "step lift," is disclosed in U.S. Pat. No. 4,466,771 to Thorley. The step lift is designed to be installed in the stairwell of a transit vehicle, and includes hinged panels that are movable between a step position and a platform position. In the step position, the hinged panels form steps for use by passengers to board and exit the vehicle. In the platform position, the hinged panels form a horizontal platform for use to raise and lower a wheelchair passenger between a vehicle floor-level position and a ground-level position. The hinged panels are attached to a carriage assembly that may be driven upward or downward to raise or lower the platform.
Typically, passengers board and exit a bus from a curb along the edge of a street. A lift generally is configured such that the carriage assembly has a lowermost point of travel that enables lowering of the platform to a street level position in order to accommodate wheelchair passengers in those instances in which boarding or exiting of the vehicle occurs from the street level rather than from a curbside. Thus, in cases where there is a curb, the height of the curb is above the lowermost point of travel of the carriage assembly. It is important to stop downward movement of the carriage assembly, i.e., lowering of the platform, at the time that the platform contacts a curb or other object. If lowering of the platform is not stopped at that time, the lift may be damaged, a passenger on the lift may be frightened and/or injured, and/or the lift may jack the vehicle.
A number of different arrangements for stopping lowering of a lift platform when the platform contacts a curb or other object have been developed. For example, U.S. Pat. No. 4,466,771 to Thorley discloses a wheelchair lift in which a sensitive edge is provided on the bottom of the platform. When the sensitive edge contacts an object, a switch is actuated to stop the motor, and hence lowering of the platform. Since it is located on the underside of the platform, the sensitive edge is susceptible to damage from ground abrasion and also to deterioration from rust, dirt, ice, or the like.
U.S. Pat. No. 4,071,152 to Kinkead et al. describes a wheelchair lift in which a ball nut engages one of the drive shafts for the platform. As the platform is lowered, the drive shaft rotates such that the ball nut is driven down the shaft. When the platform contacts an object, the drive shaft continues to rotate, driving the ball nut further down the shaft into contact with an actuator arm that activates a switch which in turn deenergizes the motor and applies the brake to stop lowering of the platform. U.S. Pat. No. 4,392,771 to Smalley describes a safety switch system for a wheelchair lift in which reciprocating hydraulic motor cylinders are mounted in vertical slots such that the cylinders are vertically movable. A projection is provided on one of the cylinders. When the platform contacts an object, the cylinders move upward in the slots, and the projection on the one cylinder actuates a safety switch to stop lowering of the platform. U.S. Pat. No. 4,407,623 to Parks describes a lift provided with a reaction switch attached to the main frame by way of bridging members held together by resilient members. When the platform contacts an object, torsional forces are transmitted to the bridging members, thereby moving the bridging members away from each other against the bias of the resilient members to engage the reaction switch. Actuation of the reaction switch terminates movement of the platform. The means for stopping lowering of the platform disclosed in these references have a relatively complex structure.
A sensing device that is protected from the environment and has a simple construction is desirable for use in detecting contact of a vehicle lift platform with the ground.